In the aircraft and aerospace industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, aluminum-lithium alloy have been developed by reason of their material properties such as low density, high strength, high fracture toughness and high modulus of elasticity.
However, continuous casting of aluminum-lithium alloys into ingot form by conventional casting processes such as direct chill casting presents problems and disadvantages including lithium burn-off, flaming, smoking and inaccessibility to the melt.
In response to these obstacles alternative direct chill continuous casting processes have been proposed to overcome these deficiencies. In U.S. Pat. No. 4,582,118 to Jacoby et al, a method for continuously casting lithium-containing alloys by a direct chill process has been proposed utilizing a fire retardant atmosphere. These prior art processes are disadvantageous in that they require an extensive and complex array of apparatus components to maintain the continuous casting operation under the fire retardant atmosphere. Moreover, operating costs are increased by the use of fire retardant materials in conjunction with the casting process.
In response to these disadvantages, a need has developed for improved aluminum-lithium continuous casting methods and apparatus which overcome these prior art deficiencies.
In response to this need, the present invention provides a method and apparatus for the direct chill casting of aluminum-lithium alloys wherein the aluminum-lithium alloys are direct chill cast under a protective molten salt flux cover comprising a mixture of lithium and potassium chloride.
In the reclamation of aluminum scrap, it is known to carry out scrap melting operations in a reverberatory or rotary furnace under a cover flux to protect the surface of the molten aluminum from oxidation and to improve the separation of the molten metal from the dross layer which forms above it. U.S. Pat. No. 4,365,993 to Meredith discloses a process for recovering aluminum from lacquer-coated scrap using a solution of a mixture of halide salts, in particular, potassium and sodium chloride. However, this patent is not concerned with the direct chill casting of aluminum-lithium alloys or the problems associated therewith.